The invention relates to centrifugal pumps, in particular, to open-impeller centrifugal pumps.
In general, such pumps comprise a driven impeller which rotates in the centre of a spiral or volute casing. The impeller may comprise a circular plate which carries a number of blades or vanes which extend outwardly from an open central area on the plate. The side of the impeller opposite the plate is open to communicate with an inlet opening in the casing.
When the impeller is driven, the rotation of the blades forces the water in the central area of the impeller outwards into the spiral casing, creating suction at the central area which draws in water along an axial direction through the inlet into the impeller.
An annular wear plate is fixed on the inside of the casing around the inlet opening and fits close against the edges of the impeller blades. The clearance between the wear plate and the impeller blades is very small and, as the impeller rotates, the edges of the impeller blades pass over the surface of the wear plate. When the pump is used to raise slurry containing sand or grit, the motion of the impeller blades over the wear plate grinds the particles suspended in the slurry into the edge of the wear plate causing wear around the inlet opening.
As the edge of the wear plate becomes worn, the clearance between the impeller blades and the wear plate increases and the pump becomes less efficient because water is able to flow back towards the centre of the impeller around the edges of the impeller blades. However, the main problem arises when the pump is started up. For convenience, such pumps are usually positioned above the level of the water being pumped and must, therefore, be capable of generating sufficient suction, when the operation begins, to raise water by a distance of several feet. Initially, the pump casing may be empty of water in which case the pump is primed by pouring water into the casing through the outlet. The clearance between the impeller blades and the wear plate is very small, for example, of the order of 0.015" (0.4 mm), so that the pump can build up a pressure difference between the inlet and the outlet sufficient to draw water into the casing. As the edge of the wear plate becomes worn, this clearance increases until the leakage of water around the edges of the impeller blades prevents the pump creating sufficient suction to draw water up to the level of the pump. When this happens, the surface of the wear plate must be refaced or the entire wear plate must be replaced. This makes such pumps relatively expensive to maintain.
These problems can be avoided to a certain extent by using a closed impeller pump in which the impeller blades extend between two parallel discs provided with inlet openings. However, such pumps are generally more expensive than open-impeller pumps and, furthermore, in some circumstances, an open-impeller is more efficient. For example, when pumping sewage or stringy substances, such as water containing weed, which do not contain abrasive solids, the chopping action of the blades of an open-impeller helps to prevent the pump becoming clogged.
Various attempts have been made to overcome the need for frequent replacement of the wear plate in open-impeller pumps. For example, the wear plate may be faced with hard tool steel. Although, the steel resists wear, it is so hard that it must be ground to produce a flat, even surface. This makes the manufacture of such wear plates expensive.
Alternatively, the edge of the wear plate around the inlet opening may be faced with a high-quality hard-wearing rubber. Although this is relatively cheap, rubber-faced wear-plates last only slightly longer than conventional wear plates of plastics or synthetic rubber. In tests in which a centrifugal pump of known type was operated continuously to draw a slurry of sand-gravel and water to a height of fifteen feet, the self-priming ability of the pump was tested at intervals. A pump having a conventional wear plate of hard plastics material (polyurethane) became unable to draw water into the pump when re-started after a period of 41/2 hours. A pump with a high quality Buna-N type rubber-faced wear plate lost its ability to draw water to a height of fifteen feet after 91/2 hours.
In some existing forms of closed impeller pumps, a seal is formed between two closely-spaced cylindrical surfaces, one surface being provided on the inlet to the impeller and the other being provided on a sleeve integral with the casing and surrounding the inlet. However, when such pumps are used to pump gritty slurries, there is a tendency for abrasive particles to become trapped in the seal and, because the radius of the seal is relatively small, the centrifugal force at the seal is insufficient to throw the particles clear of the seal. Further, as the interface between the surfaces extends parallel to the axis of rotation the centrifugal force does not act in a direction to throw gritty substances out of the seal. In addition, with this arrangement the closed impeller cannot easily be replaced with an open impeller when stringy substances are to be pumped.
It is an object of the invention to provide a centrifugal pump which is capable of pumping gritty slurries and which avoids the need for frequent replacement of the wear plate.
It is a further object of the invention to provide a centrifugal pump which is capable of pumping gritty slurries but which can be modified to pump stringy liquids merely by replacing the impeller.